Well, as the docs for FrameBuffer.getGraphics() state:
When using OpenGL support, this value is rather meaningless (but still valid).
This means: You can draw into this graphics context, but you'll never see the result. The OpenGL window is a native window of its own. Its not related to any Java2D-Component, so you can't use a Graphics-context to work with it. The Graphics-context from FrameBuffer acts as a backbuffer for some special purposes when using OpenGL...not more.
If you want to get text done in OpenGL, you have to blit it to screen yourself from a static/dynamic texture.